The SMLE No1 Mk3 was the iconic British infantry rifle of World War 1, but not the final evolution of the Lee Enfield design. By World War 2 it had been replaced by the new No4 Mk1 Lee Enfield, and this is the story of the interim models.

At the end of WW1, the British recognized several areas where the SMLE could be improved: a heavier barrel, a lighter bayonet, and aperture sights. This led to the development of the No1 Mk5 rifle (the Mk4 being a designation for a .22 rimfire training variation), with 20,000 examples made for troop trials in the mid 1920s. The Mk5 was well received by troops, with its rear-mounted aperture sight being seen as a substantial improvement over the previous tangent notch sight. However, experimentation continued and by 1926 prototypes of a Mk6 rifle were being made.

In 1929 a series of 1000 No1 Mk6 rifles was put into production, which would fit a new style of short and light spike bayonet as well as an improved type of aperture sight. They also featured a very distinctive large area of deep square checkering on the hand guard, intended to improve one’s grip on the rifle during bayonet drill. These rifles were nearly the same as what was ultimately adopted as the new No4 Mk1 rifle - so much so that in 1931 that designation was applied to the rifles and a batch of 2500 more made for trials. These trials rifles were mostly issued out to troops in the aftermath of Dunkirk, making them very scarce to find today, as most did not survive the war. Those that did will sport a new serial number with an “A” suffix to indicate their non-standard parts (in comparison to the production model No4 Mk1. Today were will look at the progressive development of a pre-prototype Mk6, Mk6 rifle number 1, a Mk6 trials rifle, and one of those 2500 trials No4 Mk1 rifles.